1. Field of Invention
The present invention relates to an optical beam splitter, especially to a fiber optic coupler with variable coupling ratio and a manufacturing method thereof.
2. Description of Related Art
A fiber optic coupler transfers optical signals from one medium such as an optical fiber to other ends so that signals from an input fiber can appear at more outputs. That means it can distribute the optical signal (power) from one fiber among two or more fibers. Thus the fiber optic coupler is applied to signal/data transmission systems such as digital subscriber line system, local area networks, and cable TV networks. However, the output ratio of conventional couplers is fixed so that its development and application are limited. Thus a fiber optic coupler with variable coupling ratio is further developed, as shown in U.S. Pat. Nos. 4,493,528 and 4,763,977.
Refer to U.S. Pat. No. 4,493,528, two fiber optic strands 11 are mounted in slots 18 formed by two bases or blocks 16 to provide proper coupling. Transducers or drivers are connected to the upper block and the lower block 16 for translating the blocks back and forth. By varying the relative positions and/or orientations of the blocks 16 and the fiber optic strands 11, the coupling ratio of the fiber optic strands 11 is variable and adjusted. Although the coupling ratio of the fiber optic coupler is variable, the manufacturing of the coupler requires high precision so that the manufacturing cost and the product price are quite high. This has negative effects on applications of the device. Moreover, the fiber optic strands 11 in the blocks 16 are easy to be affected by environmental factors such as temperature change. Thus the operation of the coupling ratio is difficult to be controlled precisely and consistently. Moreover, the input light of the coupler is with suitable wavelength so that it has relatively narrow applications. Once the operating wavelength is changed, the coupler should be changed and this is not economic and efficient.
Refer to U.S. Pat. No. 4,763,977, a coupler 14 is fabricated with fibers (1A, 1B) being mounted on a flat spring base 4 and having a bent portion in a U or S-shape. A micrometer 7 or equivalent apparatus is used to bend the spring base 4, thus straightening or otherwise stressing the bent portion in the coupler 14. The degree of energy coupled between fibers 1A and 1B is dynamically varied by the micrometer 7 so that a light spot 5 and a light spot 8 on the fibers 1A, 1B are dimming or illuminating. Yet the coupler is mounted on the spring base 4 that is driven and adjusted by the micrometer 7. The device is also under influence of environmental factors and is difficult to implement. Furthermore, only by the bent portion in a U or S-shape, the couplers have quite narrow tuning range of the coupling ratio so that they have limited applications. Thus there is no product with such design available on the market now.
The conventional fiber optic couplers with variable coupling ratio have shortcomings of unstable tuning quality of the coupling ratio, imprecise control and narrow tuning range of the coupling ratio and difficulties in implementation so that there is a need to provide new fiber optic couplers with variable coupling ratio that overcome these problems.